Local wireless communication systems typically use ultrasound, radio frequency or infrared carriers to transmit data between transmitters and receivers. These systems have scattering of the carrier which interferes with nearby transmitters and receivers. Ultrasonic and infrared systems require a line-of-sight communication channel between transmitters and receivers. The radio frequency system also has a long range because of the 1/r decay of the transmitted signal.
Magnetic inductive systems have been used in commercial applications, such as audio loops in buildings, direct speech communication through security screens, and lowrate data links from underground to equipment on the surface. The security screens typically include coils built into desk mats on both sides of the screen.
Local wireless communication systems, such as telephone systems, include a wireless audio headset worn by a user. The user's workstation communicates voice data between a telephone line and the headset. Conventional systems transmit data from the workstation over a wide area adjacent to the workstation. This data may interfere with nearby workstations.
It is desirable to have a communication system, such as a telephone headset, that may be operated in a dense office environment with low interference between adjacent work stations. It is also desirable to have a communication system that allows the user to move within a work area around the work station without a loss of or a break in the communication link.
Inductive aerials inherently amplitude modulate signals transmitted through an inductive link between the inductive aerials. At lower frequencies, changes in frequency of the communication signal due to intended modulation of the communication signal when added to the amplitude modulation from the communication link generate large percentage changes in amplitude that compromise the dynamic range of the system. In particular, this compromise includes degraded demodulating or decoding performance of a receiver.
It is desired to reduce the effects of the frequency dependent nature of the sensitivity of inductive aerials on the performance of an inductive communication link. It is desired that such reduction in frequency dependence include both the receiving and transmitting aerials. The operating frequency range of a communication system typically is controlled by a governmental regulatory agency. It is also desired the operating frequency of such a system be extended to lower frequencies to thereby use a -larger portion of the limited spectrum window.